This invention relates generally to borehole logging apparatus and methods for performing formation evaluation measurements. More particularly, this invention relates to a new and improved apparatus for effecting borehole formation evaluation, particularly nuclear logging, in real time wherein the improved formation evaluation apparatus comprises a measurement-while-drilling (MWD) tool.
Oil well logging has been known for many years and provides an oil and gas well driller with information about the particular earth formation being drilled. In conventional oil well logging, after a well has been drilled, a probe known as a sonde is lowered into the borehole and used to determine some characteristic of the formations which the well has traversed. The probe is typically a hermetically sealed steel cylinder which hangs at the end of a long cable which gives mechanical support to the sonde and provides power to the instrumentation inside the sonde. Such devices are known as wireline devices. The cable (which is attached to some sort of mobile laboratory at the surface) is also the means by which information is sent up to the surface. It thus becomes possible to measure some parameter of the earth's formations as a function of depth, that is, while the sonde is being pulled uphole. Such measurements are normally taken long after the actual drilling has taken place.
A sonde usually contains some type of source (nuclear, acoustic, or electrical) which transmits energy into the formation as well as a suitable receiver for detecting the same energy returning from the formation.
In certain formation evaluation tools, it is important to minimize the distance between the borehole wall and both the source (e.g., nuclear) and detector assemblies in the tool. For some tools, contact with the formation (i.e., borehole wall) is absolutely essential, and the quality of the measurement rapidly decreases with only a slight stand-off. For others, the reliability and/or quality of the measurement decreases with increasing stand-off, but some degree of stand-off is tolerable. This is particularly true when a measure of the stand-off is available, and the tool response can be compensated for the stand-off. Gamma-ray density tools and neutron porosity tools are examples of devices which utilize such techniques.
In order to provide contact between sensors and the formation, many of the prior art wireline formation evaluation tools are run with bow-springs. The bow-springs press the tool against one side of the borehole thereby minimizing the stand-off between the tool (sensors) and the formation face. In such cases, the sensors are mounted in the body of the tool. In other cases, tools are designed with arms, pads and mechanisms to extend the pads to the borehole wall; with the sensors being mounted in the pads. Examples of this type of prior art device are disclosed in U.S. Pat. Nos. 2,971,582 and 3,423,671.
More recently, formation evaluation tools for effecting logging in real time during drilling have become increasing popular. Such measurement-while-drilling (MWD) tools are subjected to much harsher and demanding operating conditions than prior art wireline devices. Thus, while the nuclear source and sensors in a formation evaluation MWD tool should have a minimal spacing between the tool and borehole wall as in wireline devices, it is far more difficult to effect such minimal spacing due to the restraints placed on a MWD tool.